The present invention relates to a thin-walled hollow, cylinder made from fibre-reinforced plastics material.
A hollow cylinder with a relatively great wall thickness, used as a drive shaft, is known, for example, from DE-A-29 11 167. This known cylinder is of layered structure and comprises at least four layers which contain oriented glass or carbon fibres at defined angles with respect to the longitudinal axis of the shaft. According to this publication, the innermost layer with oriented glass fibres at an angle of xc2x130xc2x0 to xc2x150xc2x0 serves as a primary support for absorbing shear loads. A first intermediate layer with oriented glass fibres at an angle of 0xc2x0 to xc2x115xc2x0 has the function of increasing the wall thickness in order to counteract torsion during use. A further intermediate layer, which contains carbon fibres with an orientation of 0xc2x0 to 15xc2x0, serves to stiffen the shaft so that the usual sound/noise from a shaft in operation can be suppressed. Finally, the shaft is provided with an outermost, impact-resistant protective layer with glass fibres oriented at an angle of xc2x160xc2x0 to 90xc2x0, which likewise contributes to resist torsion. The total wall thickness according to the examples is approximately 3.0 mm.
Furthermore, DE-A-25 20 623 has disclosed a tube made from fibre-reinforced plastics material, for example for use in the aeronautical and aerospace sectors, which comprises a plurality of layers containing optionally oriented fibres. EP-A-0 058 783 has disclosed a tubular body which comprises a plurality of plastics layers and with an outermost finishing layer, for example a mast, in which it is preferable for a combination of different fibres to be used.
A thin-walled hollow cylinder is described, for example, in NL-A-8802144, which is used to produce a so-called xe2x80x9cgalvanoxe2x80x9d printing forme, perforations in the form of a grid of dots being formed in the cylinder wall by means of a laser. This known cylinder is made from plastics material to which solid particles and/or fibres may be added in order to increase the mechanical stability and strength. Galvano printing formes made from plastics material of this nature have never achieved commercial success.
In addition to the abovementioned use for the production of an galvano printing forme, thin-walled hollow cylinders, often made from metal such as Ni, are also used for other printing applications, for example for relief printing and gravure printing, in which a relief comprising recesses is made into the outer surface of the cylinder, for example with the aid of a laser, or for use as a thin metal support for printing plates for flexographic printing which can be fitted on and removed from an air roller. Other applications for thin-walled hollow cylinders of this nature include, inter alia, the use as a transfer medium for transferring coatings and inks, an electrically conductive cylinder as an electroforming mould, as a coating material, for example as a surface with specific frictional/sliding properties, on an optionally driven roller.
In a number of these applications, it is important that it be possible to process the material of the cylinder with the aid of a laser, which means that the wall thickness must not exceed a defined upper limit. Furthermore, in some printing applications, in which the cylinder is not supported over the entire circumferential surface, but rather only at the ends of the cylinder, for example in end rings, but also in other applications, a thin cylinder of this nature has to be rigid (in the longitudinal direction), certainly if the lengths are relatively great, so that the cylinder is easy to handle, is not damageable and the bending of this cylinder is within acceptable limits.
Furthermore, it is difficult to float nickel cylinders of small diameter on rollers in which the cylinder diameter is increased with the aid of air, such as for example in flexographic printing. Nickel cylinders with a small diameter of this nature are also relatively difficult to produce. In addition, thin nickel cylinders are susceptible to creasing. However, cylinders with a greater wall thickness and therefore a reduced susceptibility to creasing are expensive and more difficult to float.
Thin-walled metal cylinders are often transported and stored with the cylinders pushed together in the shape of a kidney. However, this is only possible from a certain diameter.
Consequently, there is ongoing demand for cylinders with properties which are improved for the intended application, transport and (temporary) storage.
As has already been suggested, in accordance with NL-A-8802144, plastics can generally be made more mechanically stable by the addition of fillers and/or fibres. However, this Dutch patent application does not provide any insight into how the intended increase in mechanical stability can be achieved.
It is noted that U.S. Pat. No. 3,981,237 has disclosed a rotary printing screen made from plastics material, such as polyester. The plastics material may be filled with a reinforcing material such as glass fibres. Such a rotary printing screen is manufactured from a tubular blank made by conventional extrusion methods, which is stretched biaxially afterwards. These filler materials are (short) chopped fibres, which are distributed randomly in the plastics material, even after biaxially stretching. Accordingly the volume concentration of the fibers is 40% at maximum. However, the strength and rigidity of a screen thus manufactured leaves much to be desired. The tubular blank may be perforated by laser radiation to provide the screen openings.
Furthermore it is noted that JP-A-11 278817 has disclosed a cylinder structure made from a carbon fibre-reinforced material. This cylinder structure has a relatively thick wall in view of its application as an insulating cylinder in an apparatus for the production of silicon single crystals using the Czochralski process.
The other publications mentioned above disclose relatively thick-walled hollow cylinders which are not suitable for the abovementioned applications.
In the prior art, it is generally thought to be impossible to produce hollow cylinders from fibre-reinforced plastics material with a relatively great diameter and a small wall thickness (at most approximately 1 mm) which nevertheless have the required mechanical and chemical properties for the abovementioned applications.
Furthermore, it is known that under load plastics materials are deformed more easily than metal, such as nickel, on account of creep.
Accordingly there is a need to provide a thin-walled hollow cylinder made from fibre-reinforced plastics material, in which the wall thickness of the cylinder is such that it can be processed as an intermediate product with the aid of high-energy radiation, while the rigidity is certainly sufficient for numerous other applications, the cylinder being used with optional support and/or with optional rotation.
Also the need exists to provide a thin-walled hollow cylinder of this type made from fibre-reinforced plastics material of this nature, the mechanical properties of which are improved compared to the printing forme made from (fibre-reinforced) plastics material described in the abovementioned Dutch patent application 8802144.
The invention provides a thin-walled hollow cylinder made from fibre-reinforced plastics material, comprising at least one layer having fibres with at least one oriented direction of the fibres, the total wall thickness dtot lying in the range from 0.010 to 1 mm (10 to 1000 xcexcm), and the ratio dtot/D being xe2x89xa60.0025, where D is the diameter (in mm) of the cylinder. According to the invention, it has proven possible to create a very thin sleeve with a maximum wall thickness of 1 mm which has a continuous outer surface which can be made smooth and which can be produced within accurate dimensional tolerances, and for which little material is required. As an aside, it is noted that further machining will in many cases be unnecessary.
Preferably the total wall thickness dtot is within the range of 0.010 to 0.700 mm, more preferably within the range of 0.020-0.300 mm.
For example, at a wall thickness of 80 xcexcm, the minimum diameter is 32 mm, and at a wall thickness of 1000 xcexcm the minimum diameter of the cylinder is 400 mm. The cylinder is preferably circular in cross section and has a D/L ratio of  less than 1, where L is the length (in mm) of the cylinder.
A lightweight cylinder according to the invention is eminently suitable for use as a sleeve on a rapidly rotating roll or roller, such as for offset printing (1000 rpm), without this sleeve coming off the roller as a result of the centrifugal force. By suitably selecting plastics material(s) and fibre type(s), which have no yield point or a high yield point, it is possible to produce cylinders which are not susceptible to creasing. Compared to metal, in particular nickel, plastics are generally more corrosion-resistant and better able to withstand chemicals, which is advantageous if a cylinder according to the invention is used in aggressive media, such as when printing. Given the ratio of wall thickness to diameter which is defined above, it is possible for a plurality of cylinders according to the invention which are not being used to be transported and stored with ease in the shape of a kidney. The cylinders can easily be produced seamlessly, so that there are no inhomogeneities caused by a weld seam. Compared to metal cylinders, operations such as cutting and perforating are easy to carry out with the cylinders according to the invention.
A single plastics layer with fibres incorporated therein which are oriented in one direction (also referred to below as unidirectional fibres) has anisotropic elastic properties, i.e. the properties are dependent on the direction in which the load is acting.
In this specification a fibre-reinforced plastics material is meant to be a plastics matrix layer, wherein reinforcing fibres are incorporated. According to the invention these fibres are oriented in one direction.
It is known that the addition of fibres generally imparts an improved rigidity to plastics materials. Fibres may be added in the form of so-called short fibres (also known as xe2x80x9cchopped fibresxe2x80x9d), as long fibres which are arbitrarily distributed in the plastics material, and as unidirectional fibres. The use of unidirectional fibres in principle provides the highest rigidity which can be achieved. Furthermore, the highest fibre content can be achieved with layers having unidirectional fibres, and consequently it is with these fibres that the highest modulus of elasticity can be attained.
The invention also relates to the use of a cylinder according to the invention as a preform for the production of a printing forme for rotary screen printing, as a sleeve for flexographic and offset printing, which is to be mounted on an air roller, as a printing forme for gravure printing, as a transfer medium for transferring coatings and inks, as an electroforming mould or as a coating material.